Are you talking about bbPress or megabb? For bbPress, the keymaster (admin) password is randomly generated and displayed in the last step of the installation. It is also emailed to the email address you entered when configuring the installation.

In any case, if you have direct access to your database, you can insert a password hash for a known password value, then log in using that password for the admin account. I think that’s your best bet if you don’t know the admin password.